One method of testing for defects in articles of manufacture is to compare the article with a desired image of the object or with another object that is known to be satisfactory. A typical example is the inspection of wafers to be used in semiconductor fabrication. A slice of a wafer in process is compared to a reference, which may be a reference slice of a known good wafer, or to a desired image.
Various methods of pattern comparison have been developed. Typically, correlation techniques are used, where a signal representing a reference image is compared to a signal representing the article under inspection. In an optical correlation system, the correlation peak, which indicates a match between two images, is represented by a bright point of light in the output light distribution.
For optical applications with coherent light, where lenses may be used to produce Fourier transform images, correlation systems are often based on Fourier optics. These systems involve optical operations on transform images, which may be expressed mathematically as functions, f(x,y) and transform functions, F(u,v). The Fourier transform of the correlation is the same as multiplication of the Fourier transform of one function by the complex conjugate of the Fourier transform of the other.
One conventional approach to coherent optical correlation of two images is the use of Fourier holograms along a 4f system. The 4f system is so called because the light follows a path along a single axis that is four times the focal length of the lenses used. A photographic hologram recording is made that represents the Fourier transform of a reference object's light distribution. The hologram acts as a spatial filter, whose transmittance is the complex conjugate of incoming light. Then when light representing a first function passes through the hologram, it is modulated by the complex conjugate of the function representing the image from which the hologram was made. This output light is inverse Fourier transformed by a second lens to provide the correlation output. If the images are matched, a bright spot appears.
One problem with holographic methods that use photographic holograms is that the hologram is fixed for a particular reference object. If the reference changes, another hologram must be generated, which is costly in terms of time and money.
Correlation with spatial light modulators provides an alternative to holographic methods that use photographic recordings. In effect, the spatial light modulator acts as a real time, programmable, hologram. In an exemplary system, an input image is Fourier transformed with a lens to a transform plane, where an LCD type of transmitting spatial light modulator represents a reference image. The total light transmitted through the spatial light modulator is again Fourier transformed to result in the transform product. Although an advantage of many spatial light modulators is their programmability, problems with using them arise from their low resolution and slow write-use-erase rates. Other difficulties arise because of the sensitivity of Fourier transform operations to scale and alignment differences between image planes.
Variations of the above described techniques include joint transform systems, in which two images are presented to two different input planes. Both are Fourier transformed by the same transform lens and their Fourier product falls on the same plane. However, these systems typically include photographic holograms or spatial light modulators as holographic filters and have the problems associated with the other systems described above.
A need exists for an optical correlation system that is programmable and can respond to a changing reference in real time.